Colon cancer is the third leading cause of cancer deaths in the United States. This research focuses on the role of the Src tyrosine kinase in colonic carcinogenesis. Others and the PI have shown that downregulation of Src activity is important for differentiation and that upregulation of Src activity is important for malignant transformation of intestinal cells. Thus, the goal is to define molecular mechanisms that downregulate Src in normal colon and those that upregulate Src in colon cancer. The hypothesis is that specific domains of Src, and the proteins that bind to them, are important regulators of Src activity. Thus, effort is directed towards identifying cellular proteins that modulate Src function during intestinal cell maturation and malignant transformation. Using a yeast two-hybrid assay, the PI recently identified RACK1, a receptor for activated C kinase and a homolog of the beta subunit of G proteins, as a novel Src SH2-binding protein. The PI found that RACK1 inhibits the specific activity of Src and the growth of NIH 3T3 cells. RACK1 exerts its effect on cell growth, in part, by prolonging the G0/G1 phase of the cell cycle. The PI will further characterize Src's partner RACK1 and the mechanism by which RACK1 functions to regulate Src activity and intestinal cell growth. One aim is to assess the requirement of binding of the two proteins and of phosphorylation of RACK1, by Src for RACK1 inhibition of Src activity and cell growth. The second aim is to analyze the mechanism by which cross talk occurs between the RACK1-linked signaling pathways of Src and PKC. The third aim is to assess RACKl's influence on cell transformation by v-Src. The fourth aim is to further analyze the effect of RACK1 on the cell cycle and on Src activity during the cell cycle. These studies should generate significant new information regarding a novel inhibitor of Src and cell growth. Understanding how inhibitors of mitogenic signals work to regulate intestinal cell growth and how loss of that inhibition results in uncontrolled growth and malignant transformation, should impact our basic understanding colon cancer biology and lead to development of novel strategies for colon cancer therapy. Endogenous inhibitors of oncogenic kinases are potentially tumor suppressors; they represent exciting new targets for therapeutic intervention.